The wet press process is used for making paver units, concrete slabs (flags), and curb (kerb) stones, as well as other shaped objects. First, a mold is filled with a wet concrete mix comprising cement binder and aggregates. Next, hydraulic pressure is applied to remove water from the mix and to consolidate the mix into a compressed shape, which very often is flat (e.g., slab or kerb stone). The unit, immediately after pressing, is strong enough to be removed from the mold. The unit can be placed on a conveyor belt or shipping pallet and allowed to cure overnight.
Unlike typical precast operations, the wet-press process does not ascribe high criticality to water quantity in the concrete mix, because free water is expelled from the mix by hydraulic pressing, while a vacuum device can be employed to facilitate water drainage, such that liberal amounts of water may be used initially, so as to ensure that the concrete mix flows easily into the mold, and completely fills out the corners and edges of the mold.
However, it is important to the structure of the pressed slab or kerb unit that the aggregates be free of contaminants (such as clays); and that dust or “fines” of sufficient quality and uniformity be employed in the concrete mix to obtain a smooth, close-textured surface on the finished unit.
Typically, a paper or porous filter material is used on the bottom of the mold as well as top (or former) of the mold, to permit water to be expelled during pressing while retaining the fine aggregates and cement paste on the surface of the slab. The drainage of water from the concrete mix can be facilitated by a vacuum system. Depending upon the aggregates and conditions, the pouring/pressing/de-molding/stacking process can take place in between 15 and 30 seconds. The duration of the pressing and water drainage of the molded unit can take place in a much shorter time, typically 10 seconds or less.
To achieve highest productivity within confined manufacturing space, wet press manufacturers often employ turntable style station machines (carousel) for accomplishing the mold-filling, pressing/drainage, and de-molding phases. Each wet-pressed slab unit is removed while in the green state from the mold and placed onto its edge, upon which it stands next to (but not physically touching) another edge-standing pressed concrete slab or kerb stone unit. In this manner, the spaced apart concrete units cure using minimum space.
The present inventors refer to this ability of the de-molded green state slabs to be stood on edge next to other edge-standing green state slabs as (vertical) “stack-ability.” The vertical “stack” resembles a series of spaced-apart dominoes, which could fall over onto each other, if and when one of the concrete slabs starts to sag due to overly short pressing time, poor consolidation, or weak internal structure.
While concrete admixtures have been evaluated in attempts to improve production or concrete quality in wet press processes, admixture use is rare because none have been found to provide substantial and consistent results. Conventional water-reducing admixtures appear to increase paste viscosity and actually increase pressing time, harming the production process. While some surfactants and/or viscosity modifying agents can reduce pressing time, they have also been found to affect the green strength required for handling the pressed units after removal from the mold and/or to reduce ultimate strength of the concrete unit.
Thus, the present inventors believe that there is a long-standing need in the industry for a wet press method and admixture composition for modifying concrete rheology and internal structure of wet press units, which reduces the pressing time while improving compaction and wet state strength as well as ultimate strength of the units, and provides improved surfaces on the de-molded pressed units.